Hi! I'll be glad to be of help with this issue.
I can imagine how frustrating this situation must be for you. You are clearly a loving and caring mom and your daughter is indeed acting in very, very extreme ways. It's amazing that you're still this patient after this much time.
What is so extraordinary here is that separation anxiety really does seem like the only diagnosis available. But it is obviously more than that. However, there does not seem to be any developmental aspect to this: she can easily change and control her behavior at will, like at school and grandparents' house. That's not usual with developmental issues. So we have an attachment disorder here. And that's very unusual with you having left for only 6 weeks at age 2. I don't know what may have happened while you were away, though if anything dramatic occurred, I'm sure you would have been informed. So it was the separation from you that long ago that is driving this. Remarkable.
But that leaves you with this awful situation. Given that it's not a standard attachment disorder issue, I urge you to treat the problem with behavioral therapy. I think you need to do this both with a therapist and at home. This is now so entrenched in her that she most likely has no clue why she behaves the way she does when she's with you and at home.
The principles of Behavioral Therapy are rather straightforward: you reward the behavior you want to have her do and you don't reward the behavior you want her to avoid. Punishing bad behavior will most likely not succeed, it seems. She's way too entrenched after all this time in her fears and behavior patterns. She will most likely just take the punishments and keep on doing the same behaviors. I imagine this is what you've been experiencing.
Therefore, rewarding the desired behavior is the part of BT you want to work with her in. It is indeed easy and so can be deceiving. That's why I'd urge you to work with an experienced child psychologist or psychotherapist whose focus is in Behavioral Therapy. You need to make the rewards varied and appropriate to the behavior you are encouraging and you need to have the guidance on how to institute behavioral change techniques.
If her doctor doesn't have a good referral, here is the web address for Psychology Today's therapist directory. You can sort by zip codes and when you see someone who seems like they might be helpful (they show you a photo of the therapist!) look at the listing and see if they are experienced with children and work in BT or CBT.
Okay, I wish you the very best!
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